By way of background, the basic operation or construction of a hard disk drive has not changed materially since its introduction in the 1950s, although various individual components have since been improved or optimized. Hard drives typically contain one or more double-sided platters. These platters are mounted vertically on a common axle and rotated at a constant angular velocity by a spindle motor. During physical low-level formatting, the recording media are divided into tracks, which are single lines of concentric circles. There is a similar arrangement of tracks on each platter surface, with each vertical group of quasi-aligned tracks constituting separate cylinders. Each track is divided into sectors, which are arc-shaped segments having a defined data capacity.
Under the current iteration, each platter surface features a corresponding giant-magnetoresistive (GMR) read/write head, with the heads singly or dually attached by separate arms to a rotary voice-coil actuator. The arms are pivotably mounted to a vertical actuator shaft and connected to the shaft through a common carrier device. The common carrier device, or rack, functions as a single-movement mechanism, or comb. This actuator design physically prevents the arms from moving independently and only allows the arms to move radially across the platter surfaces in unison. As a consequence, the read/write heads are unable to simultaneously occupy different tracks or cylinders on separate platter surfaces.
A rotary actuator unitarily rotates its arms to particular tracks or cylinders using an electromagnetic voice-coil-motor system. In a typical voice-coil-motor system, an electromagnetic coil is affixed to the base of the head rack, with a stationary magnet positioned adjacent to the coil fixture. Actuation of the carrier device is accomplished by applying various magnitudes of current to the electromagnetic coil. In response to the application of current, the coil attracts or repels the stationary magnet through resulting electromagnetic forces. This action causes the arms to pivot unitarily along the axis of the actuator shaft and rotate radially across corresponding platter surfaces to particular tracks or cylinders.
A head disk assembly (HDA) houses the platters, spindle motor, and actuator mechanism. The head disk assembly is a sealed compartment containing an air-filtration system comprising barometric and recirculation filters. The primary purpose of the head disk assembly is to provide a substantially contamination-free environment for proper drive operation.
The electronic architecture of the drive is contained on a printed circuit board, which is mounted to the drive chassis below the head disk assembly. The printed circuit board contains an integrated microcontroller, read/write (RW) controller, voice-coil-motor (VCM) controller, and other standard logic circuits and auxiliary chips. The microcontroller, RW controller, and VCM controller are typically application-specific integrated circuits, or ASICs, that perform a multitude of functions in cooperation with one another. The RW controller, for example, is connected to the read/write heads (through write-driver and preamplification circuitry) and is responsible for processing and executing read or write commands. The VCM controller is connected to the actuator mechanism (through the electromagnetic coil) and is responsible for manipulating and positioning the actuator arms during read or write operations. The microcontroller is interconnected to the foregoing circuitry and is generally responsible for providing supervisory and substantive processing services to the RW and VCM controllers under the direction of firmware located on an integrated or separate EEPROM memory chip.
Although industry standards exist, drive manufacturers generally implement custom logic configurations for different hard-drive product lines. Accordingly, notwithstanding the prevalent use of extendible core electronic architecture and common firmware and ASICs, such custom logic configurations prevent printed circuit boards from being substituted within drives across different brands or models.
Cylinders and tracks are numbered from the circumference of the platters toward the center beginning with 0. Heads and platter surfaces are numbered from the bottom head or platter surface toward the top, also beginning with 0. Sectors are numbered from the start of each track toward the end beginning with 1, with the sectors in different tracks numbered anew using the same logical pattern.
Although it is often stated that tracks within respective cylinders are aligned vertically, tracks within each cylinder are actually not aligned with such precision as to render them completely perpendicular. This vertical misalignment of the tracks occurs as a result of imprecise servo writing, latitudinal formatting differences, mechanical hysteresis, nonuniform thermal expansion and contraction of the platters, and other factors. Because these causes of track misalignment are especially influential given the high track densities of current drives, tracks are unlikely to be exactly vertically aligned within a particular cylinder. From a technical standpoint, then, it can accurately be stated that tracks within a cylinder are quasi-aligned; that is, different tracks within a cylinder can be accessed sequentially by the read/write heads without substantial radial movement of the carrier device, but, it follows, some radial movement (usually several microns) is frequently required.
As a result of its common-carrier and single-coil actuator design, core electronic architecture, and vertical track-alignment discrepancy, current drive configurations prevent data from being written simultaneously to different tracks within identical or separate cylinders. In contrast, current drives write data sequentially in a successive pattern generally giving preference to the lowest cylinder, head, and sector numbers. Pursuant to this pattern, for example, data are written sequentially to progressively ascending head and sector numbers within the lowest available cylinder number until that cylinder is filled, in which case the process begins anew starting with the first head and sector numbers within the next adjacent cylinder. Because tracks within a given cylinder are quasi-aligned, this pattern has the primary effect of reducing the seek time required by the read/write heads for sequentially accessing successive data.
Hard disk-drives occupy a pivotal role in computer operation, providing a reliable means for nonvolatile storage and retrieval of crucial data. To date, while areal density (gigabits per square inch) continues to grow rapidly, increases in data transfer rates (megabytes per second) have remained relatively modest. Hard drives are currently as much as 100 times slower than random-access memory and 1000 times slower than processor on-die cache memory. Within the context of computer operation, these factors present a well-recognized dilemma: In a world of multi-gigahertz microprocessors and double-data-rate memory, hard drives constitute a major bottleneck in data transportation and processing, thus severely limiting overall computer performance.
One solution to increase the read/write speed of disk storage is to install two or more hard drives as a Redundant Array of Independent Disks, or RAID, using a Level 0 specification, as defined and adopted by the RAID Advisory Board. RAID 0 distributes data across two or more hard drives via striping. In a two-drive RAID 0 array, for example, the striping process entails writing one bit or block of data to one drive, the next bit or block to the other drive, the third bit or block to the first drive, and so on, with data being written to the respective drives simultaneously. Because half as much data is being written to (and subsequently accessed from) two drives simultaneously, RAID 0 doubles potential data transfer rates in a two-drive array. Further increases in potential data transfer rates generally scale proportionally higher with the inclusion into the array of additional drives.
Traditional RAID 0, however, presents numerous disadvantages over standard single-drive configurations. Since RAID 0 employs two or more separate drives, its implementation doubles or multiplies correspondingly the probability of sustaining a drive failure. Its implementation also increases to the same degree the amount of power consumption, space displacement, weight occupation, noise generation, heat production, and hardware costs as compared to ordinary single-drive configurations. Accordingly, RAID 0 is not suitable for use in laptop or notebook computers and is only employed in supercomputers, mainframes, storage subsystems, and high-end desktops, servers, and workstations.